This invention relates generally to magnetic resonance spectroscopy imaging (MRSI) and, more particularly, the invention relates to a method and apparatus for rapid data acquisition in MRSI.
Nuclear magnetic resonance (NMR) spectroscopy is a method that is used to study the structure and dynamics of molecules. It is completely non-invasive and does not involve ionizing radiation. In very general terms, nuclear magnetic moments are excited at specific spin precession frequencies which are proportional to the local magnetic field. The radio-frequency signals resulting from the precession of these spins are received using pickup coils.
Referring to the drawing, FIG. 1A is a perspective view partially in section illustrating coil apparatus in a MR imaging system, and FIGS. 1B-1D illustrate field gradients which can be produced in the apparatus of FIG. 1A. This apparatus is discussed by Hinshaw and Lent, "An Introduction to NMR Imaging: From the Block Equation to the Imaging Equation," Proceedings of the IEEE, Vol. 71, No. 3, March 1983, pp. 338-350. Briefly, the uniform static field B.sub.0 is generated by the magnet comprising the coil pair 10. A gradient field G.sub.x is generated by a complex gradient coil set which can be wound on the cylinder 12. An RF field B.sub.1 is generated by a saddle coil 14. A body undergoing imaging would be positioned along the Z axis within the saddle coil 14.
In FIG. 1B an X gradient field is shown which is parallel to the static field B.sub.0 and varies linearly with distance along the X axis but does not vary with distance along the Y or Z axes. FIGS. 1C and 1D are similar representations of the Y gradient and Z gradient fields, respectively.
FIG. 2 is a functional block diagram of the imaging apparatus as disclosed in "NMR--A Perspective on Imaging," General Electric Company, 1982. A computer 20 is programmed to control the operation of the NMR apparatus and process FID signals detected therefrom. The gradient field is energized by a gradient amplifier 22, and the RF coils for impressing an RF magnetic moment at the Larmor frequency is controlled by the transmitter 24 and the RF coils 26. After the selected nuclei have been flipped, the RF coils 26 are employed to detect the FID signal which is passed to the receiver 28 and thence through digitizer 30 for processing by computer 20.
Spectroscopic imaging studies have traditionally been carried out with phase encoding techniques. These techniques sample the data on a rectilinear grid, and can be reconstructed directly by a fast Fourier transform (FFT). However, phase encoded acquisitions place severe restrictions on the relationship between scan time, resolution, and signal-to-noise ratio (SNR). The SNR of the metabolites of interest is very low, and thus it is of essence to design the acquisition for maximum SNR.
The present invention provides rapid data acquisition using spiral-bound k-space trajectories to spectroscopic imaging. In contrast to conventional MRSI acquisition methods, the invention allows independent control over imaging time and spatial resolution.